It is well known to create rock fractures artificially by introducing a fluid under high pressure for the purpose of disclosing gas- or oil-bearing rock formations. To prevent re-closure of such fractures it is known to pump retaining means into the fractures, which after release of the pressure on the fractures producing fluid, keep the fissure in at least partly open condition, so that gas or a fluid such as crude oil can flow through the fractures to a bore hole. However, since resistance to flow is especially pronounced in the case of extended fractures, the length of such fractures should not exceed a certain optimum. To produce uniform disclosure, bore holes must be arranged rather close to one another, with the result that costs are considerably increased.
These disadvantages become especially noticeable where in situ gasification of coal or oil is required, either because mining of the coal is uneconomical or because the oil cannot be transported because of its high viscosity.
In the case of such in situ gasification, a basic prerequisite is the supply of oxygen to the combustion zone through an artificially created fractures (if the initial permeability of a coal seam or of a pay zone is insufficient), and the simultaneous recovery of the gas produced by the partial combustion, also through the fracture.
In view of these requirements, it has already been attempted to link two bore holes by a fracture, so that one hole could be used for oxygen supply and the other for gas recovery. However, even when the distance between the holes is small, e.g., 50 m, it is difficult to achieve accurate linkage between them. Moreover, the application of heat, steam, injection of hot water or chemical solvents, in situ combustion or gasification for improved hydrocarbon recovery, especially in low permeability formations or coal seams, requires prior preparation, which can be achieved by straightforward or advanced processes (see, e.g., U.S. Pat. No. 3,933,205).
Attempts have also been made to disclose subterranean formations by means of explosives. German Published Application No. 1 962 260 describes a process for breaking open a gas or oil reservoir disclosed by a production probe in which slurry of high force explosive is placed into the seam and is detonated. The explosive is located only in the bore hole, so that the effect of the detonation is restricted to a limited area immediately around the bore hole. Such a process is not suitable for the extension of a fracture because the amounts of explosive which can be placed into a fracture are too small, and the surrounding rock is too inert to permit permanent enlargement of a rock fracture. Even if such an enlargement were produced, it would be only temporary, since the rock pressure would promptly re-seal the fracture.
German Pat. No. 512,955 discloses an explosion process in which an aluminothermic mixture within a waterproofed casing is placed in a bore hole, water being arranged around the casing. After ignition of the aluminothermic mixture, great heat is released, causing the surrounding water to evaporate and superheat. The resulting vapor pressure causes scattering of the bore hole walls. This known explosion process is not suitable for enlargement of rock cavities such as rock fractures, because in this case the main objective is not the destruction of the fracture walls, but rather enlargement of the fracture width. In addition, the known explosion process is limited to shallow depths.